An exemplary implementation of contactless applications relies on a technology of NFC type (for ‘Near Field Communication’). These contactless applications may relate, for example, to electronic transactions in the banking sector or else in public transports, or else also identification and/or access control applications. A communication of NFC type is established between two contactless entities, or else two NFC entities, one operating in the guise of contactless card and the other operating in the guise of contactless card reader. In another mode of operation, termed the P2P mode (for ‘Peer To Peer’), two NFC entities operating in the guise of contactless card exchange data locally and therefore play an equivalent role.
Such an NFC entity is composed of a near-field communication device, or else NFC device hereinafter, and of an associated antenna which allows it to set up a contactless communication with another NFC entity. The NFC entity which operates in the guise of contactless card can for example correspond to an ‘RFID’ (from ‘Radio Frequency Identification’) radio tag. An NFC entity which operates in the guise of card reader can correspond for example to an electronic payment terminal or a facility for validating electronic transport tickets, or else a tag reader adapted for identifying radio tags, according to the applications considered.
The functionalities of a contactless entity such as this may be provided by a mobile telephone terminal. In this case, the mobile telephone terminal contains a subscriber identity card, or else SIM card (from ‘Subscriber Identity Module’), as well as an NFC device and its associated antenna.
In this context, applications termed “contactless”, which require a certain level of security, are customarily installed in the subscriber identity card of the mobile terminal, or “SIM” card. A communication can thus be established within the terminal between the contactless application of the subscriber identity card and the contactless device. Next, the contactless device, installed on the mobile terminal, allows a dialog by the mobile terminal, more precisely by the application concerned, with another contactless entity situated in proximity to the terminal. These mobile telephones can then be used in the guise of contactless entity for example.
The antennas which are used in this context may be of very diverse shapes, addressing especially both performance criteria and also criteria relating to the space occupied by the antenna.
Antennas for NFC devices are thus known which are of square or rectangular or else circular shape and which comprise a plurality of turns wound around one another.
In the search for an efficacious shape of NFC antenna, it is conventional to take care to preserve an empty space, of significant size in relation to the size of the antenna, at the center of the turns so as to allow the magnetic flux generated by the turns to pass through.
FIG. 1 illustrates an antenna according to the prior art. This antenna is a SONNET antenna illustrated on the site referenced hereinbelow:
http://www.sonnetsoftware.com/support/files/appnote/SAN-206A%20RFID%20Antenna%20Design%200602.pdf
It is formed of two antenna wires 11 and 12 each making three revolutions and therefore exhibits six turns. Each turn has a thickness of 0.5 mm. Two neighboring turns are spaced apart by 0.5 mm consecutively.
This antenna shape complies with the constraint mentioned hereinabove, consisting in maintaining a consistent free space in relation to the antenna as a whole for the electromagnetic flux generated. Indeed, having turns of relatively small thickness, this antenna makes it possible to preserve a space for the passage of the magnetic field within the turns.